1. Field of the Invention
This invention relates to pressure sensing transducers and more particularly to capacitive type pressure sensing transducers.
2. Description of the Prior Art
Many forms of pressure sensing transducers utilizing capacitive sensors have been disclosed in the prior art. Most variable capacitive sensors have a deformable metallic member such as a diaphragm, bellows or beam which forms one plate of a plate type capacitor and can be moved relative to a base structure by the actuating pressure. An electrode member rigidly connected to, but electrically insulated form, the base and in close proximity to the deformable member forms the other plate of the variable capacitive sensor. Conventional capacitance sensors of this type have several significant disadvantages. Proper alignment of the sensor components is very difficult to achieve. Since reasonable values of capacitance and high relative change of capacitance with plate movement require very small gap dimensions, typically on the order of 0.001 inches, proper control of parallelism and gap dimensions is extremely important. To achieve proper alignment of the capacitor plates, very precise and small tolerances must be placed upon the component parts.
The possibilities for degradation of performance by thermal shift in structural alignment is difficult to avoid with a typical capacitive type transducer having long thermal paths in the structure determining the relative portions and alignment of the capacitance plates. The thermal problems include both the changes induced by different ambient temperatures and the transient effects produced when a temperature change occurs. These problems are further complicated by the fact that structure requirements for particular portions, such as the elastic properties of the diaphragm material, make it difficult to choose materials such that there may be a cancellation of the effects caused by thermal exapansion. A further disadvantage of existing structures is that mounting stresses, which frequently occur when the structure is affixed to the system being measured, cause distortions in one or both of the capacitance plate and support structures. Such distortions can cause a shift in the initial value of the capacitive sensor and/or a change in the rate at which capacitance changes with applied force.
Capacitive pressure sensing transducers have also been constructed of ceramic, quartz or other dielectric materials to form chambers or walls with conductive films on their interior surfaces. U.S. Pat. Nos. 3,715,638 and 3,858,097 granted to W. R. Polye are illustrative of such constructions. The operative portions of these prior art constructions are substantially flat and of substantially uniform thickness. With chambers or capsules having walls of uniform thickness there is a stress concentration in the peripheral region where the walls are fused together and the deflection of the conductive surfaces of the transducer varies with the radial position of the deflection portion. In U.S. Pat. No. 4,168,518 granted to S. Y. Lee there is disclosed a capacitive pressure transducer structure in which deflection and maximum stress is controlled by the elastic properties and strength of the dielectric material carrying the capacitive plates rather than by the properties of a fusing or cementing material.
As previously related, prior art capacitive type pressure or force sensing transducers have been found to be sensitive to temperature change. They also have high impedance output and frequently require complex external electronic circuitry. The typical capacitive type transducers utilize a single plate type capacitor system and must be reactively as well as resistively balanced with external capacitive bridge circuitry. Long lead lengths and moving leads allow stray capacitive impedance pickup and thus introduce extraneous impedance variations to the detection, measurement and pressure or force value indication circuitry. It is often necessary to have a preamplifier close to the transducer.